Problem: A bag contains $4$ red marbles, $9$ green marbles, and $7$ blue marbles. If a marble is randomly chosen, what is the probability that it is red?
Explanation: There are $4 + 9 + 7 = 20$ marbles in the bag. There are $4$ red marbles. The probability is $ \frac{4}{20} = \dfrac{1}{5}$.